The present invention relates to casting processes for the manufacture metal and alloy structures, and more particularly, to systems, methods and materials associated with casting using fugitive patterns.
Investment casting, which is also commonly known as the lost wax process, is one of the oldest known techniques utilized for forming metal. In practice, a wax substance is patterned, such as by injecting melted wax into a die. When the wax cools, it solidifies in a pattern that corresponds to the component to be cast in metal. The molded wax pattern then is invested in a ceramic shell mold, such as by repeatedly dipping the wax pattern in a ceramic slurry until a desired thickness of ceramic material is built-up on the wax pattern. The shell mold is initially heated to remove the molded wax pattern and is then fired at an elevated temperature to develop appropriate mold strength for casting a molten metal or alloy.
There are several major limitations and potential problems with using wax to manufacture investment castings, especially when it comes to certain components, such as highly complex castings, large castings and/or castings with relatively thin cross-sections. For example, the dimensional accuracy of a part manufactured using a conventional investment casting process is limited due to the inherent distortion of the pattern that occurs in the processing of wax. Moreover, wax must be handled at room temperature or lower to prevent undesired wax characteristics and/or defects, such as shape distortion, fingerprints, creep, etc. Still further, conventional wax costs are relatively high due to the requirement of initial melt cycle(s) necessary to melt the wax prior to injecting the wax into a corresponding die. Cost is further increased due to the relatively high scrap rates, which typically results due to metal leakage from shell mold cracks caused by wax expansion and due to defects that lead to inclusions in the molten metal caused by ash content from wax reacting with the shell material.
Although investment casting techniques have existed for thousands of years, attempts to find suitable alternatives to wax have been largely unsuccessful. For example, attempts to form fugitive patterns from polystyrene or polyurethane foam have not been successful in high volume commercial investment casting operations, due for example, to problems with pattern surface quality, pattern material strength, cycle time and difficulty in removal of the pattern from the corresponding shell mold.